A major product in the acid catalysed dehydration of glucose or cellulose is 5-hydroxymethyl-2-furfuraldehyde, also known as hydroxymethyl furfural (HMF). The structure of HMF is shown below:
HMF represents one key intermediate substance for substitute of petrochemicals and is readily accessible from renewable sources like carbohydrates. Energy consumption throughout the world has steadily risen over the last century and is expected to increase at a faster rate due to the rapid development throughout the world. Thus, in order to meet this growing energy demand of the world, lignocellulose biomass has come up as a strong candidate for the production of various valuable petroleum products. Experts throughout the world consider lignocellulose biomass as the only sustainable source of clean energy which can meet the current world growing energy demand. Lignocellulosic biomass has the potential to displace the petroleum derived product, which is presently used as transportation fuels. Now-a-days, the researchers have only used model biomass compounds (fructose, glucose, inulin, etc), that do not contain the impurities present in biomass feed stocks for preparation of chemicals. A suitable green technology for the production of various useful bio-chemicals directly from biomass has not been developed so far.
India produced more than 6.0 million tons per annum of aromatic spent biomass. Some of the major aromatic crops cultivated in India includes mentha, citronella, lemongrass, pamarosa, patchouli, etc. These aromatic crops contain 4-8 w/w % of essential oil. After extraction of the valuable essential oil by steam or hydrodistillation method, the spent biomass is of no use and treated as waste material. People generally used this spent distilled biomass for burning purpose which also leads to environment related problems. This patent describes the development of sustainable, integrated and holistic strategies for the production of various valuable bio-chemical. The major constituent present in aromatic spent biomass is cellulose (35-40%), hemicellulose (25-30%) and lignin (15-20%). Cellulose is a major biopolymer of glucose unit, is used for synthesis of HMF. It is known that, the high boiling point of HMF (291° C.) is the limitation factor to be used as fuel. But, HMF can be used for the production of 2,5-dimethyl furan (DMF) which has a high calorific value. Also, DMF has the lowest water solubility and the highest research octane number or RON (106) among mono-oxygenated C6 compounds. Its physicochemical properties are competitive to ethanol. Its energy density (31.5 MJ/l) is 40% higher than ethanol (23 MJ/l) and much closer to gasoline (35 MJ/l). DMF has better anti-knock quality than gasoline. It has higher boiling point (92° C.) than ethanol (78° C.), which made it less volatile and an ideal factor for liquid transportation fuel. HMF can be used for the production of various valuable bio-chemicals like chiral reagents, biologically active materials, polyhydroxyalkanoates, polymers and polymerization initiators, antifouling compounds, personal care products, lubricants, adsorbents, printing inks, coatings, electronics, photography, batteries, drug delivery, corrosion inhibitors, bio-pesticides, etc.
Aromatic spent biomasses represent one of the most abundant and underutilized biological resources on the planet. From the olden times, these biomasses were simply used to burn and generate thermal energy. Nowadays, utilization of these lignocellulosic biomasses can produce liquid bio-fuels and value added chemicals.
Many researchers around the world have tried and produced HMF from various sources of carbohydrates like glucose, fructose, inulin, sucrose etc. These food grade carbohydrates (sugars) are easily soluble in water. It is straightforward to convert these carbohydrate solutions directly into HMF. Some researchers have reported the use of high boiling point solvents such as butanol, methyl isobutyl ketone, dimethyl acetamide, dimethyl formamide, DMSO etc to produce HMF from sugar solution. Some researchers have also mentioned the process of treating the sugar solution with metal chlorides such as CrCl2, CrCl3, CuCl2, FeCl2, RuCl2 etc or Amberlyst resins or Dowex resins or mineral acids as catalyst at high temperature (120 to 150° C.) for synthesis of HMF. But very limited researchers around the world are able to produce HMF from lignocellulosic waste biomass.
Carbohydrates like fructose are very easily converted into HMF. Fructose easily gets dissolved in various organic and aqueous solvents and follows a single step (dehydration) for conversion to HMF. A study lead by Ilgen et al., (Green Chem. 2009, 11, 1948) reported the conversion of fructose to HMF. In their study, they have used choline chloride-citric acid (ChCl-CA) solution for carrying out the reaction. Fructose being the simplest monosaccharide, it gets converted to HMF even in the absence of catalyst in high-boiling solvents such as DMSO, DMF and DMA. It is reported that some food grade carbohydrates (fructose, glucose, sucrose, etc) also contained HMF as degraded chemical in heat processing of these reducing carbohydrates, which indirectly determines the inferior quality of food product. Very limited researchers around the world are able to produce HMF from lignocellulosic waste biomass. In our patented process, we have developed a novel eutectic solvent medium for cellulose dissolution and a novel hybrid catalyst for carrying out the reaction.
Many researchers have tried to use expensive imidazolium chloride based ionic liquids (ILs) for cellulose dissolution and HMF production. D Argyropoulos in his patented study (WO/2008/098036) reported the use of acid catalyst and expensive imidazolium chloride based ILs for carrying out the thermolysis process of lignocellulosic biomass. But this process is not commercially viable due to the expensive nature of ILs. Another study lead by Jong et al. group (WO2011149341) reported the production of HMF from lignocellulosic biomass using a combination of expensive ILs and organic acid. This process is also not commercially viable due to the expensive nature of ILs and recoverable limitation of organic acid as catalyst. Su et. al. (Applied Catalysis A; 361, 2009, 117) also reported the use of expensive imidazolium chloride based ILs for cellulose dissolution. In their study, they reported the use of ILs for dissolving the pure cellulose and higher yield of HMF. Though this process offered appreciable yield of HMF, but it cannot be practiced in the industrial scale due to expensive ILs. Another study lead by Dhepe et al., (WO/2011/092711) group describes single step hydrolytic process for converting lignocellulose mainly xylan (xylose) into furfural. In their study, they have used common organic solvent (alcohol, ether, ester, hexane, acids, toluene, xylene) to dissolve the hemicellulose (xylan) and expensive dowex resin as catalyst for carrying out the reaction. In this process, they mainly targeted the easily soluble hemicellulose for the furfural production and failed to explain the dissolution of cellulose in a solvent system and the production of HMF. In their process, they have claimed, 40% yield of furfural production from xylose, obtained from hemicellulose.
Some of the researchers have also used different combination of solvents dimethyl acetamide-LiCl and expensive imidazolium chloride based ionic liquid for conversion to HMF. Study leads by Binder and Raines (J. Am. Chem. Soc. 131, 2009, 1979) reported the use of dimethyl acetamide-LiCl solvent system for cellulose dissolution and HMF production. In this process, they have used low cost solvents but the main constraint arises in this system is the isolation of HMF from the reaction mixture. It has been reported that, the physical and chemical properties of both the dimethyl acetamide and HMF are very close together (having very close boiling point). Hence, it is very difficult to separate the product HMF from the reaction solution i.e dimethyl acetamide. Similarly, LiCl is also soluble in most of the extracting solvents and again needs extensive isolation process for getting the reasonable pure HMF. This separation problem of HMF added cost to this process. Apart from isolation difficulties, using this solvent the reasonable yield of HMF is possible only with an addition of imidazolium chloride ILs in DMA-LiCl system. The use of ILs along with DMA-LiCl, make this process expensive. All this processes discussed above are expensive and having separation limitation.
Background of the Preparation of 5-Hydroxymethyl Furfural
HMF and 2,5-disubstituted furanic derivatives have great potential in the field of intermediate chemicals from the re-growing resources. Due to their various functionalities, HMF could be utilized for producing wide range of products. Most of the works throughout the world have reported on the synthesis of HMF from fructose. HMF represents one key intermediate substance and can be used as a suitable starting source for the manufacture of various furan monomers. The reaction mechanism for synthesis of HMF follows cyclic fructofuranosyl intermediate pathways. Regardless, the mechanism of HMF formation, the intermediate species formed during the reaction may in turn undergo further reactions such as condensation, rehydration, reversion and other rearrangements, resulting in a plethora of unwanted side products.
Although preparation of HMF has been known for many years, a method which provides HMF with good selectivity and in high yields has yet to be discovered. The complications arose from the rehydration of HMF, which yielded the by-products such as levulinic acid and formic acid. Another unwanted side reaction included the polymerization of HMF and/or fructose resulting in humin polymers, which were solid waste products. Further complications could arise as a result of solvent selection. Water is easy to dispose of and dissolves fructose, but unfortunately low selectivity and increased formation of polymers and humin increased under aqueous conditions.
Agricultural raw materials such as cellulose, sucrose or inulin are inexpensive starting materials for the manufacture of hexoses (glucose and fructose). As discussed above, these hexoses could be converted to HMF. The dehydration of sugars to produce HMF is well known. HMF was initially prepared in 1895 from levulose by Dull (Chem. Ztg., 19, 216) and from sucrose by Kiermayer (Chem. Ztg., 19, 1003). However, these initial syntheses were not practical methods for commercial scale production of HMF due to very low conversion of the starting material. Commonly used catalysts for the preparation of HMF includes corrosive inorganic acids such as H2SO4, H3PO4, and HCl, etc. These acid catalysts were used in solution and are very difficult to regenerate and dispose. In order to avoid this regeneration and disposal problems, solid catalysts have been used in the present process.
The purification of HMF has also proved to be a troublesome operation. On long exposure to high temperatures at which the desired product can be distilled, HMF and impurities associated with the synthetic mixture tend to form the degradation products. Because of this heat instability, a falling film vacuum still must be used. Even in such an apparatus, resinous solids form on the heating surface causing a stalling in the rotor and frequent shut down time making the operation inefficient. Prior work has been performed with distillation and the addition of a non-volatile solvent like PEG-600 to prevent the buildup of solid humin polymers. On the other hand, the use of polyglycols leads to the formation of HMF-PEG ethers. The prior art processes also fail to provide a method for producing HMF that can be performed economically.
Although preparation of HMF has been known since 1895, but a method which provides HMF with eco-friendly route, good selectivity and high yields has not been developed so far.
Following are some of the most important processes known related to the production of HMF:                (a) Most of the processes claim the synthesis of the HMF from glucose or fructose or inulin or sucrose using high boiling point solvents such as butanol, methyl isobutyl ketone, dimethyl acetamide, dimethyl formamide, DMSO etc. The sugar solution was treated with metal chlorides such as CrCl2, CrCl3, CuCl2, FeCl2, RuCl2 etc or Amberlyst resins or Dowex resins or mineral acids as catalyst at high temperature (120 to 150° C.) for synthesis of HMF. The dehydration of fructose is quite facile; even in the absence of catalyst in high-boiling solvents such as DMSO, DMF and DMA, whereas glucose requires a special catalyst for the formation of HMF. Therefore, the synthesis of HMF from food derived products is a very easy process.        On the other hand, conversion of fructose to HMF in choline chloride-citric acid (ChCl-CA) has been reported (Ilgen et al., Green Chem., 11, 1948). But no work on synthesis of HMF from cellulose (Complicated plant based biopolymer) using this combination has been reported. Structurally, fructose and cellulose are entirely different molecule. Cellulose having a stable regular long chain polysaccharides of glucose molecules link by β-1,4 glycoside linkage. Hence it is very difficult to dissolve and break this β-1,4 glycoside linkage of cellulose molecule. Whereas fructose is a simple monosaccharide, which is easily dissolve in various organic or aqueous solvents and follows only one step process for conversion to HMF.        (b) Some of the processes claim the use of imidazolium based ionic liquids such as 1-ethyl-3-methyl imidazolium chloride, 1-butyl-3-methyl imidazolium chloride, 1-ethyl-3-methyl imidazolium acetate, 1-ethyl-3-methyl imidazolium hydrogen chloride, 1-butyl-3-methyl imidazolium acetate, 1-butyl-3-methyl imidazolium hydrogen chloride, 1-ethyl-3-methyl imidazolium hydrogen sulphate and 1-butyl-3-methyl imidazolium hydrogen sulphate, etc. for dissolving the microcrystalline cellulose. Then, the cellulose solution is treated with metal chlorides such as CrCl2, CrCl3, CuCl2, FeCl2, RuCl2, etc or mineral acids or zeolite or Amberlyst, Dowex resins as catalyst at high temperature (120 to 150° C.) for synthesis of HMF. Su et al. (Appl. Catal. 2009, 361, 117) have prepared the HMF by using combination CrCl2 and CuCl2 catalysts. But, they used the expensive ionic liquid (1-ethyl-3-methyl imidazolium chloride) as the reaction medium for dissolution of cellulose.        (c) Limited reports are available on the use of mixture of dimethyl acetamide-LiCl (DMA-LiCl) for dissolving the cellulose. Mechanistic analyses reveal that loosely ion-paired halide ions in DMA-LiCl are critical for weakening the cellulosic β-1,4-glycosidic linkages. Though, DMA-LiCl is able to solubilise the cellulose but conversion of cellulose to HMF is poor in this solvent system. Binder and Raines (J. Am. Chem. Soc. 2009, 131. 1979) have reported the synthesis of HMF by using DMA-LiCl solvent system along with 1-ethyl-3-methyl imidazolium chloride using metal chlorides such as CrCl2, CrCl3, etc or mineral acids as catalyst.        
In the hitherto known processes, especially the first category (a) is feasible because the sugar solutions (monosaccharides or disaccharides) are easily soluble in aqueous or polar organic solvents. The HMF is also synthesized from these reduced sugars in mild conditions with high conversion ratio. Second category (b) process needs very expensive imidazolium based ionic liquids for dissolving the cellulose. Though it provides appreciable yield of HMF, but the process cannot be practiced in the industrial scale due to expensive ionic liquid. In the third category (c) process, the cellulose is dissolved by using cheap solvent (DMA-LiCl) system, but it has the separation limitation of reaction product (HMF) from the solvent system. The appreciable yield is only obtained, when DMA-LiCl along with the imidazolium ionic liquid is used as the reaction medium, but major constraint of the process is isolation of HMF from the reaction medium. It is known that the physical and chemical properties of both dimethyl acetamide and HMF are very close together. Also, both solvents boiling point are very high, so it is very difficult and also quite expensive to separate the product (HMF) from the reaction solution (dimethyl acetamide). Similarly, LiCl is also soluble in most of the extracting solvents and again needs extensive isolation process for getting the reasonably pure HMF. Finally, the third category process is not completely free from ionic liquid solvent. From the above literature survey, it is clear that imidazolium based ionic liquids or DMA-LiCl system can solubilise the cellulose, but former one is costly process and later one is reasonably costly along with separation (HMF) limitations.
The catalyst system reported for synthesis of HMF are metal chlorides such as CrCl2, CrCl3, CuCl2, FeCl2, RuCl2, AlCl3 etc or mineral acids or zeolite or Amberlyst, Dowex resins.